Three-dimensional density fields of the twin jets were numerically and experimentally investigated. The present study focused on the comparison of the density distribution for the twin jets. The results obtained by the computational fluid dynamics (CFD) and three-dimensional background-oriented schlieren (3D-BOS) indicate that the periodic density fluctuation appears in the potential core each nozzle, and the flow structure of the twin jets is quite similar. The distribution of the normalized density value at the nozzle centerline agrees well with CFD and 3D-BOS. The density value of the shear layer between the nozzles increases as the interaction of the twin jets occurs. The trend of increasing and decreasing the interference between the nozzles was almost the same as each other. On the other hand, the position where the interaction of the twin jets starts and the growth rate of interaction were different. This is probably due to the effect of the laminar-to-turbulent transition occurred in the results of CFD. This result indicates that the laminar-to-turbulent transition can be estimated from the velocity fields obtained by CFD and particle image velocimetry (PIV).

Comparison of three-dimensional density distribution of numerical and experimental analysis for twin jets

A simple noninterferometric optical probe is developed to estimate wavefront distortion suffered by a plane wave in its passage through density variations in a hypersonic flow obstructed by a test model in a typical shock tunnel. The probe has a plane light wave trans-illuminating the flow and casting a shadow of a continuous-tone sinusoidal grating. Through a geometrical optics, eikonal approximation to the distorted wavefront, a bilinear approximation to it is related to the location-dependent shift (distortion) suffered by the grating, which can be read out space-continuously from the projected grating image. The processing of the grating shadow is done through an efficient Fourier fringe analysis scheme, either with a windowed or global Fourier transform (WFT and FT). For comparison, wavefront slopes are also estimated from shadows of random-dot patterns, processed through cross correlation. The measured slopes are suitably unwrapped by using a discrete cosine transform (DCT)-based phase unwrapping procedure, and also through iterative procedures. The unwrapped phase information is used in an iterative scheme, for a full quantitative recovery of density distribution in the shock around the model, through refraction tomographic inversion. Hypersonic flow field parameters around a missile-shaped body at a free-stream Mach number of ∼8 measured using this technique are compared with the numerically estimated values. It is shown that, while processing a wavefront with small space-bandwidth product (SBP) the FT inversion gave accurate results with computational efficiency; computation-intensive WFT was needed for similar results when dealing with larger SBP wavefronts.

Improved quantitative visualization of hypervelocity flow through wavefront estimation based on shadow casting of sinusoidal gratings.

Three-dimensional density measurement of unsteady flow field around a sphere is carried out in the ballistic range at Institute of Fluid Science, Tohoku University. Simultaneous multi-angle measurement system using twelve digital cameras is installed in the test chamber of the ballistic range to achieve the three-dimensional density measurement. The Colored-Grid Background Oriented Schlieren (CGBOS) technique using colored-grid background is utilized for the reconstruction of density. The Mach number of the sphere is set to 1.5. The short pulse LEDs to illuminate the backgrounds are also installed in the chamber to capture the unsteady flow field around a flying sphere. Three-dimensional density distribution around a sphere is successfully reconstructed.

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The Quantitative Density Measurement of Unsteady Flow around the Projectile

Closure to “Discussion of ‘A Three Dimensional Density Field Measurement of Transonic Flow From a Square Nozzle Using Holographic Interferometry’” (1977, ASME J. Fluids Eng., 99, pp. 743–744)

The invention provides a visualization method for supersonic two-dimensional flow field data, and the method comprises the following steps: a, building the mapping relation between flow field densityvalues and RGB colors, and obtaining the distribution of RGB colors of grid units through interpolation; b, taking the dimensionless density as the height values of mesh points, obtaining a three-dimensional density field curved surface similar to a water flow free surface, and calculating the illumination intensity of each grid unit; c, multiplying the RGB colors of each grid unit by the illumination intensity, and obtaining the final color distribution. According to the scheme of the invention, the method can achieve the capturing of a shock wave structure with the severe spatial changes anda spiral structure with the more gentle changes, achieves the presentation of flow field density distribution information of a uniform area, and achieves the high-fidelity display of a two-dimensional supersonic flow field.

Visualization method for supersonic two-dimensional flow field data

The colored-grid background oriented schlieren (CGBOS) technique is applied to the measurement of the interaction field of lateral jet and cross-flow. The test model is designed based on DLR’s generic model. Experiments were carried out in supersonic blow-down wind tunnel of JAXA/ISAS with 0.6 m × 0.6 m test section. Free-stream Mach number is set to 2.0. The quantitative experimental data obtained by CGBOS technique and the reconstruction results of the density for the interaction field between cross-flow and lateral jet are described. Unsteady phenomena in the lateral jet/cross-flow interaction field are also measured using color high-speed camera. The growth of the interaction field is reconstructed and visualized three dimensionally.

Quantitative density measurement of the lateral jet/cross-flow interaction field by colored-grid background oriented schlieren (CGBOS) technique

Laser Interferometric Computed Tomography (LICT) measurement is practically useful method to measure quantitatively supersonic unsteady flow field, and threedimensional (3D) density distribution is obtained. In our previous study, the 3D flow fields have been observed by LICT measurement. However, it is still difficult to measure 3D density distribution of the different times in CT reconstruction technique (4D-CT). In this study, to realize the novel 4D-CT measurements, high-speed camera is applied to our LICT measurement. This is because successive images at sufficiently short intervals can be obtained by high-speed camera.

Application of High-Speed Camera to 4D-CT Density Measurement of Unsteady Shock-Vortex Flow Discharged from Two Inclined and Cylindrical Holes

The study of shock wave is of significance in understanding supersonic flow. In this paper the threedimensional (3D) quantitative measurement unsteady and discharging of shock waves is described. Threedimensional shock-vortex flow field has been investigated by Laser Interferometric Computed Tomography (LICT) measurement with a diaphragmless shock tube in our laboratory. The complex flow field induced by discharging unsteady shock waves from three cylindrical holes can be successfully measured by pulsed LICT with Algebraic Reconstruction Technique (ART), and results are discussed.

3D Laser Interferometric CT Measurement of Unsteady Shock-Vortex Flow Discharged from Three Cylindrical Holes

The Background Oriented Schlieren (BOS) technique proposed by Meier [1] enables us to take a quantitative density measurement of a flow field with computeraided image analysis. In the past several years, the BOS technique has been applied in various experiments [2],[3].

Quantitative Density Measurement of the Interaction Field of Side Jet and Cross Flow by Colored-Grid Background Oriented Schlieren (CGBOS) Technique

The measurement of unsteady shock-vortex flow fields can be done only qualitatively by two-dimensional (2D) visualization methods, such as shadowgraph, colorschlieren, quantitatively by 2D interferometric images.

Yoshihiro Miwa

Papers

Quantitative density measurement of the lateral jet/cross-flow interaction field by colored-grid background oriented schlieren (CGBOS) technique

Application of High-Speed Camera to 4D-CT Density Measurement of Unsteady Shock-Vortex Flow Discharged from Two Inclined and Cylindrical Holes

3D Laser Interferometric CT Measurement of Unsteady Shock-Vortex Flow Discharged from Three Cylindrical Holes

Quantitative Density Measurement of the Interaction Field of Side Jet and Cross Flow by Colored-Grid Background Oriented Schlieren (CGBOS) Technique

Four-Dimensional LICT Density Measurement of Unsteady Shock-Vortex Flow Discharged from Two Inclined and Cylindrical Holes Using High-Speed Camera